1. Field of the Invention
The present invention relates to a differential dipole antenna system adapted for applications of transmission/reception of differential signals with wide bandwidth. It also relates to a corresponding transmission and/or reception device.
2. Discussion of the Background
Radiofrequency transmission/reception systems supplied by differential electrical signals are very attractive for present and future wireless communications systems, particularly for concepts of autonomous communicating objects. A differential supply is a supply by two signals of equal amplitude in phase opposition. It contributes to reducing, or even eliminating, the noise known as “common mode noise”, undesirable in transmission and reception systems.
In the field of mobile telephony for example, when a non differential system is used, a significant degradation of the radiation performance is indeed observed when the operator holds a handset provided with such a system. This degradation is caused by the variation, due to the hand of the operator, of the distribution of current on the frame of the handset used as ground plane. The use of a differential supply makes the system symmetrical and thus reduces the concentration of current on the casing of the handset: it thus renders the handset less sensitive to common mode noise introduced by the hand of the operator.
In the field of antennas, a non differential supply leads to undesirable radiation of a crossed component due to the common mode circulating in the non symmetrical supply cables. The use of a differential supply eliminates crossed radiation from the measurement cables and thus makes it possible to obtain reproducible measurements, independent of the measurement context as well as perfectly symmetrical radiation diagrams.
In the field of active components, power amplifiers of “push-pull” type, the structure of which is differential, have several advantages, such as dividing in two the output power and the elimination of higher order harmonics. In reception, low noise differential amplifiers have several perspectives in terms of reduction of the noise factor. Also, the use of a differential structure prevents the undesirable triggering of oscillators by common mode noise.
The electrical dipole antenna is the differential antenna that may be envisaged the most naturally. It is an antenna constituted of two identical and symmetrical arms, supplied by two signals of equal amplitude and in phase opposition. Recently, thick dipoles known for their wide bandwidths have been widely used for high speed communications, in accordance with the different UWB (Ultra Wide Band) communication standards aimed at communications with wide bandwidths. When they are used in non symmetrical devices, these antennas show problems of common mode noise, the differential supply of which makes it possible to overcome.
For reasons of optimisation of their size, these antennas are moreover advantageously formed using coplanar technology, particularly using differential CPS (CoPlanar Stripline) technology. Furthermore, differential CPS technology makes it possible to benefit from the advantages of differential structures while enabling simple coplanar integration with discrete constituents: it is not necessary to create via type connections to link the constituents together. The absence of ground plane also makes it possible to envisage a simple connection, less perturbing with other differential coplanar constituents. Consequently, more and more differential devices are designed according to this technology.
The invention thus more specifically relates to an antenna that comprises, on a same surface of a dielectric substrate, a first half of a thick radiating dipole, a first conducting strip of a bi-strip line for supplying a differential signal, the first conducting strip being connected to the first half of the thick radiating dipole, a second half of a thick radiating dipole and a second conducting strip of the bi-strip supply line, said second conducting strip being connected to the second half of the thick radiating dipole.
Such a differential dipole antenna is for example described in the document “Differential and single ended elliptical antennas for 3.1-10.6 GHz ultra wideband communication”, of Powell et al., IEEE Antennas and Propagation Society International Symposium Proceedings, vol. 3, pages 2935-2938 (2004). In this document, the thick dipole comprises two radiating halves of elliptic shape supplied by a differential bi-strip line. It ensures operation in a range of frequencies ranging from 3.1 to 10.6 GHz for UWB type applications. In particular, the WiMedia UWB standard allocates bandwidths between 4.2 and 4.8 GHz in Europe, to ensure compatibility with American standards. An elliptic differential dipole antenna of this type is also described in the document “Planar elliptical element ultra-wideband dipole antenna”, of Schantz, IEEE Antennas and Propagation Society International Symposium Proceedings, vol. 3, pages 44-47 (2002).
In the document “A novel CPS-fed balanced wideband dipole for ultra-wideband applications”, of Chan et al., Proceedings of the European Conference on Antennas and Propagation, EuCAP 2006, pages 235.1 (2006), the thick dipole comprises two radiating halves of half disc shape supplied by two conducting strips of a differential bi-strip line.
More generally, “thick dipole” is taken to mean any dipole in which the radiating halves occupy a compact geometric surface, such as a polygon (in particular a triangle), an ellipse, a disc, a half ellipse or a half disc.
It may also be noted that the more a dipole antenna is thick and has slow transition of field lines between its arms, the more it has a wide bandwidth. Several geometric shapes make it possible to attain more or less wide bandwidths. For example, a “butterfly” type antenna, the arms of which are of triangular shape, has a relative bandwidth, defined by the relation Δf/f0 where Δf is the width of the bandwidth and f0 the central operating frequency of the antenna, of the order of 20%. An elliptic antenna may, in certain cases, have a relative bandwidth exceeding 100%.
The aforementioned antennas are quite compact and with wide bandwidth but they generally have the dimension of a half wave at the low operating frequency, i.e. 30 to 40 mm at 4 GHz. In numerous applications where a very high miniaturisation is required, they remain however too bulky. In particular, applications generally targeted are those using USB wireless type communication protocols, on USB cards of very small sizes for which the dimensions cited above are not suitable.
Unfortunately, most of the known conventional miniaturisation techniques are not valid for coplanar differential symmetrical structures. Furthermore, the laws of physics and electromagnetism provide a reduction in bandwidth with the reduction in the size of the antennas, which is not desirable, particularly in the aforementioned applications.
Furthermore, an antenna must generally be connected to a band pass filtering device. Indeed, an antenna is a device that transmits and receives electromagnetic power. A band pass filter is then used to limit the frequency band in which the antenna is going to transmit or receive electromagnetic signals. This makes it possible to reduce the noise captured out of band and to prevent interference of signals transmitted or received by the antenna with signals transmitted by other communications systems operating on other sometimes neighbouring frequency bands.
In a conventional manner, filters manufactured independently are connected to the antennas. This requires in most cases the use of matching circuits or instead long transitions, costly in terms of size and losses added to the overall system.
To reduce the dimensions of a filtering antenna system and improve its efficiency, the European patent application published under the number EP 1 548 872 provides forming a filtering antenna using multilayer technology. In this document, the radiating constituent of the antenna is placed on an upper layer and a coupled resonator filter is formed on a multiplicity of lower layers of the structure between the radiating structure and a ground plane. However, although compact, this filtering antenna has a narrow bandwidth on account of the use of a patch type antenna. In addition, its formation requires mastery of multilayer technology, which is quite costly and difficult to put in place.
In fact, few works have tackled the integration of an antenna and a filter using differential technology. However, the formation of an integrated set of filtering antenna using differential technology makes it possible to connect it directly to the active circuits, generally also formed using differential technology, and thus to do away with line-balance converter circuits (or baluns) which increase the cost and the size of a transmission/reception system and reduce its efficiency.
Such a differential wide band filtering antenna is nevertheless described in the document “Co-designed CPS UWB filter-antenna system” of Yang et al., IEEE Antennas Propagation International Symposium Proceedings, June 2007, pages 1433-1436. This filtering antenna is formed using differential CPS technology. In addition, the filtering device of this antenna ensures the impedance matching of the high impedance loop antenna used. This differential filtering antenna thus has several advantages, such as the elimination of impedance matching circuits and the elimination of baluns.
However, apart from the fact that the filtering device of this antenna ensures the impedance matching and the symmetrization of the loop antenna, there is not really any joint design of these two constituents since, neither the antenna which is an ordinary loop antenna, nor the filter which is formed by rectilinear conducting strips with impedance jump, are optimised in terms of size. Indeed, the filtering antenna assembly formed in this document occupies a large size, of the order of a guided wavelength, which makes it difficult to integrate it in current portable telecommunications systems.